1. R. J. Charlson is in the Department of Atmospheric Sciences and Department of Chemistry, University of Washington, Seattle, WA 98195, USA.
2. J. H. Seinfeld is in the Department of Chemical Engineering and the Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA 91125, USA.
3. A. Nenes is in the Department of Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA.
4. M. Kulmala is in the Department of Physics, University of Helsinki, 00014 Helsinki, Finland.
5. A. Laaksonen is in the Department of Applied Physics, University of Kuopio, 70211 Kuopio, Finland.
6. M. C. Facchini is at the Instituto di Scienze dell' Atmosfera e dell' Oceano-CNR, via Gobetti 101, 40129 Bologna, Italy.